1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive employing a multiple-input sequence detector responsive to reliability metrics for improving a retry operation.
2. Description of the Prior Art
Disk drives for computer systems typically execute retry operations when a data block recorded on a disk is unrecoverable during an initial read operation due to a low signal-to-noise ratio (SNR) in the read signal which prevents accurate detection of the data. Because the magnitude and character of the noise during any given read operation can vary, rereading the data block during retry operations may result in at least one read operation where the SNR is high enough to accurately recover the data block. However, it may require numerous retry operations to successfully recover the data block which can increase the latency of the disk drive as it must wait for the disk to complete a revolution with each retry. Furthermore, the SNR may never be high enough to enable recovery of the data block regardless as to the number of retries, which means the data block is permanently lost. Losing data blocks becomes more problematic as the SNR decreases due to the increasing demand for disk drives with higher areal storage density.
Modulation and error correction codes (ECC) are typically employed in disk drives in order to increase the effective SNR and thereby improve the reliability and decrease the latency involved with retry operations. Examples of modulation codes include a run length limited (RLL) code which limits the spacing between adjacent surface alterations on the disk in order to limit intersymbol interference (ISI), a distortion in the read signal caused by closely spaced, overlapping pulses. For example, in a system where a binary xe2x80x9c1xe2x80x9d bit modulates a surface alteration and a xe2x80x9c0xe2x80x9d bit modulates no surface alteration (i.e., NRZI recording), an RLL (d,k) code constrains the recorded data sequence such that at least d xe2x80x9c0xe2x80x9d bits occur between consecutive xe2x80x9c1xe2x80x9d bits, thereby ensuring that consecutive surface alterations are spread apart to limit ISI. Other examples of modulation codes include trellis codes, matched spectral null codes, and other codes directed at increasing the effective SNR.
Modulation codes are typically augmented by ECC codes which further increase the effective SNR by encoding the user data into codewords that exhibit a minimum distance property measured relative to a Hamming distance. The Hamming distance defines the difference between valid codewords of the ECC code, and the minimum Hamming distance defines the correction power of the ECC code.
The extent that modulation and ECC codes increase the effective SNR is referred to as the xe2x80x9ccoding gainxe2x80x9d, which is normally measured as the SNR difference (in dB) between a system with coding and a system without coding that will achieve a specified bit error rate. There is a limit, however, to the amount of gain that modulation and ECC codes can provide in a storage system because of the additional redundancy required to implement the code which decreases the user data density. This ratio of user data bits to codeword bits is referred to as the code rate; as the code rate decreases, the channel density must increase in order to maintain a desired user data density. There is a true coding gain only if the code rate is large enough to allow an increase in the user data density as compared to an uncoded system. Thus, the extent that modulation and ECC codes increase the effective SNR and thereby improve the reliability and decrease the latency involved with retry operations is limited.
Other techniques have also been employed in disk drives in order to increase the effective SNR and thereby improve retry operations. As mentioned above, ISI typically causes the SNR in the read signal to decrease as the areal density increases. In addition to RLL codes, various filtering techniques have been employed in the prior art to slim the pulses in order to reduce the undesirable degradation caused by ISI, but filtering the read signal tends to boost the high frequency noise. More recent disk drives employ special signal processing techniques referred to as partial response (PR) equalization with maximum likelihood (ML) sequence detection or PRML sequence detection which allows for a controlled amount of ISI rather than attempting to eradicate it through filtering. Since the effect of the controlled ISI in PRML systems is known, it can be taken into account in the sequence detection algorithm when demodulating the read signal into an estimated data sequence. This increases the effective SNR and thereby improves the reliability and decreases the latency involved with retry operations; however, the extent that known PRML systems improve performance during retries is limited.
There is, therefore, a need for a disk drive with an enhanced capability of accurately decoding a data block deemed unrecoverable during an initial read operation by improving a retry operation. In particular, there is a need to reduce the number of retries and associated latency required to accurately recover a data block, as well as to improve retries in order to recover an otherwise unrecoverable data block.
The present invention may be regarded as a disk drive employing an improved retry operation. The disk drive comprises a disk for storing a data block, and a head for reading the data block to generate an analog read signal during an initial read operation. A sampler samples the analog read signal to generate a sequence of read signal sample values, and a multiple-input sequence detector detects an initial estimated data sequence from the read signal sample values during the initial read operation. A reliability metrics generator generates reliability metrics representing an estimated reliability for data in the initial estimated data sequence, wherein the reliability metrics are stored in a local memory. During a retry operation, a memory controller transfers the reliability metrics generated during the initial read operation to the multiple-input sequence detector which generates a retry estimated data sequence using the reliability metrics, whereby the reliability metrics improves the probability of accurately detecting the retry estimated data sequence.
The present invention may also be regarded as a method of improving a retry operation in a disk drive. A data block stored on a disk is read to generate an analog read signal. The analog read signal is sampled to generate a sequence of read signal sample values, and an estimated data sequence is detected from the read signal sample values. Reliability metrics are generated representing an estimated reliability for data in the estimated data sequence, wherein the reliability metrics are stored in a local memory. During a retry operation, the data block is reread and the step of detecting an estimated data sequence responds to the reliability metrics generated during a previous read operation which improves the probability of accurately detecting the estimated data sequence during the retry operation.